MONDAY, Nov. 18, 2013 (HealthDay News) -- The influence of just one cigarette ad can last for seven days and increases the risk of smoking among college students, according to a new study.

Over three weeks, 134 students, aged 18 to 24, in Pittsburgh documented their exposure to cigarettes ads and other pro-smoking media messages. This included seeing positive depictions of smoking in movies, for example, the researchers noted.

The students, who included both smokers and nonsmokers, also recorded how these exposures affected their smoking intentions and ability to refuse cigarettes.

Immediately after seeing a cigarette ad or other pro-smoking media message, the students' smoking intentions rose by an average of 22 percent, the study found. Although their smoking intentions decreased with each passing day, they remained elevated for seven days.

All of the students said they were exposed to numerous smoking ads and pro-smoking media message multiple times during the three-week study. In total, the students reported more than 1,000 such exposures, according to the findings published online Nov. 18 in the Journal of Adolescent Health.

"We were surprised how long the influence of pro-smoking messages lasted," study co-author Steven Martino, a psychologist at the RAND Corporation, said in a news release from the nonprofit research organization. "The results suggest that positive media messages about smoking are likely to influence behavior even if opportunities to smoke occur infrequently," he noted.

"Our findings suggest that exposures that occur before the influence of a prior message 'wears off' could cause the risk of smoking to accumulate over the long term. This might explain why exposure to these media messages can have an enduring effect on people's attitudes and behaviors toward smoking," Martino explained.

He and his colleagues noted that the study results have important implications for policies that limit tobacco advertising and other efforts meant to reduce youth tobacco use.

Cigarette ads are banned from television and radio in the United States, but are still allowed at places where tobacco is sold, in newspapers and magazines, and on the Internet.